1. Field of the Invention
The invention relates to a vehicle power generating device. More particularly, the invention relates to a device that controls power generation in an alternator having a voltage regulator that keeps generated voltage constant, as well as to a control method of that alternator.
2. Description of the Related Art
Japanese Patent Application Publication No. 2006-149131 (JP-A-2006-149131) describes a vehicle power generating device that controls power generation in an alternator. This device includes a voltage regulator that keeps the generated voltage that is output from the alternator constant, an overheat protection circuit that protects the alternator from overheating, and a voltage control circuit that controls the voltage regulator based on the state of an on-board battery and the like.
In this device, the voltage control circuit outputs a high level control signal when the voltage of the on-board battery is lower than a target voltage, and outputs a low level control signal when that voltage is higher than the target voltage. Also, the overheat protection circuit performs an overheat protection operation in which it reduces the excitation current of the alternator when the temperature of the alternator exceeds an upper limit value of a normal range. More specifically, the overheat protection circuit outputs a PWM (Pulse Width Modulation) signal to limit the excitation current. The output from the voltage control circuit and the output from the overheat protection circuit are both input to an AND circuit, and the output from the AND circuit is input to a switching element of the voltage regulator.
In this vehicle power generating device, when the temperature of the alternator exceeds the upper limit value of the normal range, the excitation current of the alternator is reduced by controlling the switching element of the voltage regulator. Therefore, with this device, the alternator can be protected from overheating by stopping or suppressing power generation when the temperature of the alternator is abnormally high.
Typically, an overheat protection circuit uniformly reduces or stops the excitation current of the alternator by changing the applied voltage that is based on the output from the voltage control circuit to a low value when the temperature of the alternator is abnormally high. However, with the structure described above, power generation of the alternator is uniformly reduced or stopped depending on the control of the alternator itself, regardless of the state of the on-board battery or the on-board electrical load. As a result, the supply of power to the on-board system may be significantly affected, e.g., the battery may go dead unexpectedly to the controller.